Forum Schedule Spring 2026

This talk will focus on experimental and theoretical research into nonlinear molecular rovibrational spectroscopy. One focus will be on the spectroscopy of CH4 where a high power single mode OPO is used to pump individual transitions in the n3 fundamental and the resulting disequilibrium probed with near IR radiation from a either a frequency comb or a single mode cw laser. This has allowed us to probe transitions to states with ~9000 cm-1 of vibrational energy with resolution of ~1-10 MHz and frequency accuracy of ~10 kHz. We have used a modification to measure dipole moments of the E symmetry states in his region. The other focus will be on degenerate, Doppler free, two-photon transitions detected using a version of Cavity Ring-down Spectroscopy. This increases spectral resolution by about two orders of magnitude and reduces effective line density also by a few order of magnitude due to the selectivity for transitions that are nearly double resonant. In favorable cases, the sensitivity is even higher than for detection using single photon transitions of the same molecule.

The X-ray spectra of accreting supermassive black holes encode the physics of the innermost regions: the accretion disk, the broad-line region, and powerful outflows. However, two decades of CCD-resolution observations left these components blended and ambiguous. The Fe Kα emission line at 6.4 keV is the single most powerful diagnostic of this environment: produced by fluorescence when hard X-rays irradiate circumnuclear matter, its profile encodes the location, kinematics, and physical conditions of the emitting gas, from the inner accretion disk to the broad-line region to the molecular torus. XRISM/Resolve, the first X-ray microcalorimeter to observe a large sample of active galactic nuclei, changes the game: its 5 eV energy resolution in the Fe K band (resolving power of ~1300 at 6 keV) cleanly separates narrow core, broad, and Compton-shoulder components for the first time. I present an overview of the early XRISM observations of active galactic nuclei, showing how Resolve's ability to decompose the Fe K complex into its distinct physical components builds a coherent picture of the circumnuclear environment that connects, for the first time, to what we know from optical, UV, and infrared observations. I discuss constraints on ultra-fast outflows, the view into Compton-thick nuclei, and the questions this new spectral clarity opens for the future of high-resolution X-ray astrophysics.

With the latest gravitational-wave catalogs providing hundreds of new compact object mergers, the population properties of these objects are becoming an increasingly rich probe of their origins. In particular, there is growing evidence that some merging black holes are created from previous mergers of two or more smaller black holes. Using predictions for the spins of these hierarchically-merged systems, we identify a subpopulation in the gravitational-wave data consistent with a hierarchical-merger origin in dense star clusters. Surprisingly, we find that such hierarchical mergers were likely more common in the past than they are now, providing an explanation to previously-discovered correlations in the binary black hole population. In this talk, I will give an overview of what the population of gravitational wave sources has taught us thus far, discuss the redshift evolution of different contributors to this population, and show what they teach us about the sizes, formation histories, and metal content of star clusters out to cosmological distances.

Supermassive black holes are thought to play a central role in shaping the evolution of galaxies, particularly in shutting down star formation in the most massive systems. However, the pathways by which these black holes grow over cosmic time—and how that growth connects to their host galaxies—remain uncertain. A growing body of work combining cosmological simulations and observational surveys indicates that the suppression of star formation is closely linked to black hole activity from the early Universe to the present day. While this connection is now well established, direct constraints on the growth history of supermassive black holes remain limited.

X-ray reflection spectroscopy provides a unique probe of the innermost regions of accretion flows, where strong-gravity effects enable measurements of black hole angular momentum (spin). As a quantity that retains memory of past accretion and merger events, spin offers a powerful observational handle on black hole growth pathways. Recent work has begun to place these measurements in a broader cosmological context by combining spin constraints from X-ray reflection spectroscopy with predictions from galaxy formation models, enabling the first population-level tests of black hole growth scenarios. These efforts move beyond individual objects toward statistical constraints on how supermassive black holes assemble their mass. The next generation of upcoming X-ray observatories will significantly expand these measurements, enabling a direct link between small-scale accretion physics and the large-scale evolution of galaxies.